This invention relates to the production of microcrystalline semiconductors, particularly for use in semiconductor devices.
Semiconductors are useful in a wide variety of devices. Examples include memories, field effect, luminescent and thin film devices and displays.
Amorphous semiconductors, when hydrogenated, are particularly useful for photovoltaic devices which product a voltage when subjected to radiation, or radiate when electrically energized.
The first photovoltaic semiconductors were produced from relatively thick single crystals. Subsequently, amorphous material with suitable photosensitivity was fabricated by glow discharge in a gaseous atmosphere. The glow discharge process unfortunately produces defects in the resulting material by virtue of ion bombardment. Nevertheless, it was shown in 1972 that amorphous silicon could be produced by glow discharge from silane with lower defect density than anticipated by the use of controlled conditions. W. E. Spear and P. G. LeComber, J. Non-Cryst. Solids, 8-10, 727 (1972).
In 1975 Professor Spear and his co-workers demonstrated that wide ranging control over the electronic properties of glow discharge amorphous silicon could be achieved by doping with phosphorus and boron. Spear, et al., Solid-State Commun., 17, 1193, (1975).
In 1976 Carlson used glow discharge amorphous silicon to produce Schottky barrier and P-I-N junction devices. Carlson U.S. Pat. No. 4,064,521 (Dec. 20, 1977). The Carlson patent discusses P-I-N junctions with heavily doped P and N layers, P-N-N junctions with heavily doped P and N layers, as well as hetereojunctions. These devices all made use of amorphous silicon produced by glow discharge in silanes. Unfortunately the basic amorphous silicon devices disclosed by Carlson have numerous disadvantages. Thus P-I-N junctions made from amorphous silicon suffer from excessive light loss in the junction layers. Moreover, the built-in potential of such a junction is undesirably low because the Fermi levels of both the P and N materials are relatively far (0.2 to 0.3 electron volts) from their respective band edges.
It has been speculated that if the separation of the Fermi levels from the respective conduction and valence band edges can be reduced, there would be a consequent increase in built-in voltage. This would also result in an increase in the open circuit voltage of the cell formed by the use of variously doped materials, as well as an increase in the field strength of the intrinsic layer used in such cells. The result could be an improvement in carrier collection, together with the current and fill factor of the cell.
Accordingly, it is an object of the invention to increase the built-in voltage of semiconductive devices. A related object is to increase the built-in volgate of semiconductive devices fabricated from semiconductanes, including silanes and germanes. Another related object is to increase the built-in voltage of devices fabricated from amorphous semicondctors.
Another object of the invention is to decrease the light loss in semiconductor devices. A related object is to decrease light loss in the frontal layer of multi-layer semicondictive devices. Still another related object is to decrease the light loss in the frontal layer of multilayer silicon devices in which the body of the device is of intrinsic amorphous silicon.
Still another object of the invention is to enchance the doping of semiconductive layers. A related object is to decrease the ohmic contact resistance with doped semiconductor layers. Another related object is to reduce the resistive power loss in semiconductive devices, particularly those which employ a body of amorphous silicon.